Liquid crystal display devices are in widespread as display devices for portable appliances as represented by the notebook personal computers, due to the feature of thinness and light weight. In particular, the active-matrix liquid crystal display devices using active elements as represented by thin film transistor elements (TFTs) are recently becoming popular as the monitors for the desktop personal computers as well as the display terminals for office-automation appliances, etc., by virtue of the point of high image quality equivalent to the Braun tube.
However, in the liquid crystal display devices there exists a defect of narrowness of viewing angle in nature.
There is a proposal, as means for thoroughly resolving this, on the display mode called an in-plane switching mode (IPS).
Concerning the in-plane switching mode, there are disclosures, e.g. in “R. Kiefer, B. Weber, F. Windcheid and G. Baur, Proceedings of the Twelfth International Display Research Conference (Japan Display '92) pp. 547-550”.
Meanwhile, besides this, there also are descriptions in “SID 97 DIGEST p 184-187”, Japanese Patent Laid-open No. 62788/1998, Japanese Patent Laid-open No. 62802/1998, etc.
As apparent from the above documents, the liquid crystal display device utilizing an in-plane switching mode as above has pixel regions each formed with at least one pair of electrodes (not necessarily in integer but may be 1.5 pairs for 3 electrodes) on a surface of one transparent substrate on the side of a liquid crystal. Between these electrodes, an electric field is caused in a direction horizontal to the transparent substrate surface, thereby structurally controlling the amount of light transmitting between the electrodes.
However, it has reached assurance that in the liquid crystal display device thus structured, in each pixel, if examined for a characteristic of light transmissivity (equivalent to luminance) against a voltage (voltage difference) applied to the electrode, vary steeply in a middle tone range (0-20% in luminance).
This means that the light transmission amount greatly varies even with slight change in electrode-to-electrode voltage and, in the middle tone range, is easily discriminatable for the human eye.
Due to this, it has been reached necessary to improve the quality of display by balancing the characteristic at around the middle tone range with the characteristics in other ranges than the middle tone range.
Meanwhile, it is possible for each of the electrodes to obtain a predetermined optical transmissivity by making the spacing between them. However, the variation in photolithography or etching in forming the electrodes results unavoidably in variation of electrode spacing on each pixel.
In order to cope with this, countermeasures have been taken with improvement of photoresist materials, etch solutions, etc. such that the variation due to photolithography or etching upon forming the electrodes can be suppressed in maximum extent. However, the variation has increased with the increase in size of the liquid crystal display screen. Thus, this countermeasure is no longer to cope with.
The present invention solves the above problem, and it is an object to provide an active-matrix liquid crystal display device of an IPS scheme having a wide viewing angle equivalent to the Braun tube which is high in luminance uniformity and well in image quality without having unevenness of luminance.